1. Field of the Invention
This invention relates to data processing, and more particularly, but not exclusively, to methods and apparatuses for shuffling and subsequently de-shuffling digital data representing a television signal, for the purpose of making it easier to conceal errors.
2. Description of the Prior Art
There is increasing interest in the digitizing of television signals, for example, for the purpose of recording with a digital video tape recorder (VTR). To effect digitization, the composite analog video signal or component analog video signals is sampled at regular intervals along each horizontal scan line and the resulting sample values are coded into binary digital form. Then, on reproduction of the digital signal from the digital VTR, a replica of the original analog video signal can be formed.
The digital signal which is recorded comprises data words corresponding to the analog video signal samples and further words or at least bits to identify the digital signal generally, for example by time or subject, to identify the horizontal lines and fields to which the samples belong, and to enable horizontal and vertical synchronizing signals and color burst signals to be added back to the analog video signal to make a complete reproduced television signal. So far as the parts of the digital signal corresponding to the samples are concerned, if reproduction of the recorded video signal was always perfect, then these parts of the digital signal could, for example, be in the form of 8-bit words, each word representing a respective sample, and sequential words representing sequential samples along the horizontal line. In practice of course the reproduced signal is commonly less than perfect, with the result that individual bits or whole words are corrupted and, in the case of drop-out, whole sequences of words are lost completely. Errors like this are clearly detrimental to the reproduced television picture, and steps must therefore be taken to reduce the effects of such errors.
The two techniques used are error correction and error concealment. In error correction, data words which contain errors are identified and the errors corrected. To do this, additional bits are recorded simply to make provision for error correction. This increases the amount of information to be handled and increases the already high data rates, so a compromise is necessary between the number of bits added for error correction purposes and the degree of error correction achieved. One proposal is to recode each 8-bit digital word as a corresponding 10-bit digital word. By using only those 10-bit words which have substantially equal numbers of "0"s and "1"s and in which long runs of either digit are absent, the digital signal can be made more suited to the recording characteristics of magnetic tape, and the recovery of the bit rate clock frequency from the reproduced digital signal can be made easier. However, for error correction, the important point is that the additional bits enable some error words to be recognized as such.
Having so far as possible effected correction, there remains the problem of words which although recognized to be in error cannot be corrected. This can occur for example where there is a drop-out. These errors are concealed. Various concealment techniques have been proposed, but generally they rely on the availability of words corresponding to samples adjacent to the sample corresponding to the word which is in error or has been lost. As a very simple example, consider three successive samples along a horizontal line. If the word corresponding to the middle sample is in error or has been lost, a reasonable approximation to the correct sample can be made by taking the average of the samples on either side.
Unfortunately, however, error words or lost words are very likely to occur in bursts of successive words, rather than as single words. Indeed, in some error correction techniques when a word is found to be in error it is automatically assumed that several words on either side of the error word are also in error, even although they have not been positively identified as being in error. So long as the data words are recorded in the natural sequence corresponding to successive samples and horizontal lines, therefore, the use of concealment techniques is likely to be hampered by the lack of adjacent or at least near-by correct words.
To get over this problem, it has been proposed to shuffle the order of the digital words prior to recording and to de-shuffle the order after reproduction so dispersing any error words, but it is necessary to do this without unacceptably increasing the amount of or the complexity of the data handling apparatus.